Leaving aside Robins very valid comments about the extra space consumed by a QCTP and the resulting need for more tailstock travel if working between centres the fundamental disadvantage of any QTCP system is the sheer cost of all the tool holders needed for efficient working.
Proper exploitation of a QTCP requires enough holders to carry all your regularly used tools plus a spare or three to take the extras needed for the particular job in progress. Something like 10 or 12 seems to be considered a desirable aspiration if you are not to run out of ready to go tools and either have to change mid job or plan ahead adjusting the loaded set before starting.
Setting a new tool to centre height mid job when you’ve got something in the machine that you’d rather not have to remover and re-aligning can be a right pain if the job crowds out the height gauge. Shuffling tools in an inadequate number of holders gets old fast! Marking the tailstock barrel or casting to indicate centre height can be a useful work around but, in my experience, doesn’t really fill the gap if a job is mounted.
A less costly alternative is swopping out complete, loaded, block tool posts whether conventional 4 way or the, preferable in my view due to being smaller, 2 way. Such blocks are quite easily made by gluing and screwing together stock sections of plate and bar material. Steel for the top and bottom, alloy in the middle to make tapping for the screws easier seems a good way to go about things. HSS tooling can be shimmed on the bench using a height gauge, inserts just need the right thickness base or a fixed shim to get the holder in the right place. A simple drilled block does just fine for boring bar and is more solid than the usual carrier.
I used a similar block swopping system on SouthBend lathes for a fair number of years and found it adequately satisfactory. Room for improvement in my first implementation to make accurate relocation simpler but a usefully better version 2 never seemed worth the effort.
The centre spigot on your machine is an advantage in locating block posts rather than a pain when arranging for a QTCP system. You will need to make arrangements for some sort of rotational location so the blocks go back in the same orientation. Whether a simple pin in the toolpost and matching location hole in the topside catering for one position or some sort of sub base giving several options. I pretty much never rotate the tool post on my machines so would not find a fixed angle from a single hole limiting. If I do rotate the post it’s for specific cut reasons so no need for general variability.
In an ideal world a quick release thread for the holding down handle would be faster than a standard one but a normal one with a few turns to release is liveable with. Some years ago i schemed out a plausible quick release system requiring only about 1/3 rd of a turn to operate but the whole concept got sidetracked by changing machines. The new one came with a Dickson QC set up with serval tool holders so I just haunted E-Bay until I had enough holders.
Clive